9 erasers cost $17.73. Which equation would help determine the cost of 6 erasers?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 9 erasers. We want to know the cost of 6 erasers. We can write the numbers of erasers as a proportion: $\dfrac{9}{6}$ We know 9 erasers costs $17.73. We can let $x$ represent the unknown cost of 6 erasers. The proportion of these costs can be expressed as: $\dfrac{\$17.73}{x}$ The cost changes along with the number of erasers purchased, and so the two proportions are equivalent. $\dfrac{9}{6} = \dfrac{\$17.73}{x}$